We have developed a new technique for isolating cells that resist transformation by specific oncogenes using 4-cis-hydroxy-L-proline as a selective agent. We have begun to characterize additional revertants from ras-transformed cells. The new revertants are resistant to transformation by members of the ras gene family and to certain tyrosine kinase oncogenes, but they are sensitive to retransformation by mos, raf, and fos. Combining the retransformation data observed here with those from our ouabain-derived revertants isolated previously (C-11 and F-2), we find that all oncogenes can be placed into one of four groups, and we suggest that this represents a primitive functional classification. If true, the ras gene would require the role of at least two cellular 2 functions (such as attachment to the cell membrane and kinase activity) to successfully transform mouse cells. Transfection of a cDNA library prepared from one of the revertants, CHP 9CJ, has resulted in the isolation of several DNA sequences that may be associated with the revertant phenotype. One of these sequences may be identical to the K-rev-1 gene previously isolated from ras-resistant human cells.